3 Single Phase Transformers Wiring Diagram for 3 Phase

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stang623

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We have recently relocated to a new building with 480 volt power, its an older building and they seem to like using three single phase transformers to accomplish a 240/120 step down. I'm currently re purposing one of these units and wondering the benefit's of parallel versus series wiring? I am looking for the output to be a 240 volt 3 phase, 4 wire wye setup. We will be running three phase equipment and single phase 120 outlets off of this panel. Which way would you guys recommend parallel or series? Do the kva's add up in one circumstance or the other?

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It seems like you would use the series for single phase cause you would have two hots and a neutral...but not sure on three phase, its not making sense to me just yet.

If anyone had a diagram for this that would be amazing.
 

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You need to look at wiring diagrams for 240/120 3-phase 4-wire connections. This system often goes by several slang name, a popular ones are 'wild-leg' and 'high-leg'.

Before we go off into the weeds: what loading will you require? How much 3-phase 240V?, How much single phase 240V, how much single phase 120V?
 
The closest you can get to what you want is either 208Y/120v 3ph or 240D/120v 3ph with a high leg (which can't supply 120v loads).

You also must be very careful about connecting the primaries together correctly and the secondaries together correctly.
 
208Y/120 would be great...loading wise it is likely to be half 3 phase and half single phase 120, we have a couple of small 3 phase machines running off this panel and then it would be feeding three offices with outlets for computers and a small fridge.
 
In that case, you want to connect the primaries in delta: L1 to T1-H2 and T2-H1, L2 to T2-H2 and T3-H1, and L3 to T3-H2 and T1-H1, for example. (each T refers to a given transformer.)

The secondaries should all be wired for 120v like the diagram on the right, with all three X1/X3's connected together as your grounded neutral, and each X2/4 becomes a secondary line.
 
LarryFine said:
In that case, you want to connect the primaries in delta: L1 to T1-H2 and T2-H1, L2 to T2-H2 and T3-H1, and L3 to T3-H2 and T1-H1, for example. (each T refers to a given transformer.)

The secondaries should all be wired for 120v like the diagram on the right, with all three X1/X3's connected together as your grounded neutral, and each X2/4 becomes a secondary line.
Click to expand...

I had to draw that one out to visualize the connections, but that does make sense Larry.
So with the resulting setup would the overall kva be cumulative? (15 kva x 3 = 45 kva)?

How about current rating for either primary and secondary, or secondary only OCPD selection?

Thanks!
 
Larry, once I drew this out like previously suggested, it made ton's of sense...actually laminated it and stuck in in the electrical room on the wall next to the switch gear. Thanks a bunch.

I would assume that I could get 15 kva on each of the three leg's?
 
stang623 said:
Larry, once I drew this out like previously suggested, it made ton's of sense...actually laminated it and stuck in in the electrical room on the wall next to the switch gear. Thanks a bunch.

I would assume that I could get 15 kva on each of the three leg's?
Click to expand...

Frank DuVal said:
Yes. 15 kVA at 120 volts. 125 Amps per leg.
Click to expand...
Could have a total of 45 KVA of 120 volt load with all three combined.

If he went with the delta configuration, he can only have 120 volt loads connected to one side of the delta and would be limited to 15 kVA of 120 volt loads. Makes the rest of the delta useless if you use up all of one side with 120 volt loads.
 
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